Combined once-through and natural circulation vapor generator

ABSTRACT

A sodium-heated vapor generator used in connection with a sodium-cooled fast breeder reactor is provided with means for supplying feedwater to the vapor generator in the event of feed pump failure, low-load operation, or reactor shutdown. Accordingly, a check valve will complete a natural circulation circuit to provide the necessary feedwater until water can be supplied from a suitable emergency supply.

United States Patent lnventor Robert O. Barratt Parsippany, NJ.

Appl. No. 851,769

Filed July 18, 1969 Patented Oct. 19, 197 l Assignee Foster WheelerCorporation Livingston, NJ.

COMBINED ONCE-THROUGH AND NATURAL CIRCULATION VAPOR GENERATOR 3 Claims,2 Drawing Figs.

US. Cl 122/34, 122/406 Int. Cl F22b 1/06 Field of Search 122/32, 34,406, 407

T0 STEAM USER Relerences Cited UNITED STATES PATENTS 3,473,519 10/1969Hansen 122/32 3,177,659 4/1965 Berman 122/32 X 2,704,534 3/1955 Dalin etal. 122/407 Primary Examiner-Kenneth W. Sprague Attorneys-John E.Wilson, John Maier, Ill and Marvin A.

Naigur ABSTRACT: A sodium-heated vapor generator used in connection witha sodium-cooled fast breeder reactor is provided with means forsupplying feedwater to the vapor generator in the event of feed pumpfailure, low-load operation, or reactor shutdown. Accordingly, a checkvalve will complete a natural circulation circuit to provide thenecessary feedwater until water can be supplied from a suitableemergency supply.

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ROBERT O. BAR T COMBINED ONCE-THROUGH AND NATURAL CIRCULATION VAPORGENERATOR BACKGROUND OF THE INVENTION Although vapor-generating systemsof the once-through type have been widely used, there are generally anumber of drawbacks when once-through systems are used in connectionwith nuclear applications. Accordingly, when a sodium-heatedonce-through vapor generator is used with a sodium-cooled fast breederreactor, it represents the final source of cooling for the reactor, andthis source of cooling must always be available even if the main supplyof feedwater is discontinued due to failure of the feed pump or shutdownof the reactor. Thus, when the feed pump fails there must be analternate system to continue the heat transfer circuit from the nuclearreactor. This is also necessary when the reactor is shutdown, since heatis generated for a lead period which amounts to low-load operation ofthe vapor generator. When a oncethrough system is used it is notpossible to operate without external control means, and if such controlmeans are not available the ability of the system to transfer heat islost.

In accordance with the present invention, start up, shutdown, andlow-load operation of a once-through vapor generator may be accomplishedby providing a natural circulation circuit for the recirculation of aportion of the working fluid from the drum storage to the vaporgenerator, thereby providing a reliable source of cooling during periodswhen outside sources of control are lost. Thus, a principle object ofthe present invention is to provide a sodium-heated oncethrough vaporgenerator with an auxiliary natural circulation circuit so that a sourceof cooling will always be available even if the main supply of feedwaterbecomes discontinued.

SUMMARY OF THE INVENTION In accordance with an illustrative embodimentdemonstrating features and advantages of the present invention there isprovided a once-through vapor generator capable of operation undervariable load conditions and achieving indirect heat exchange betweenliquid sodium and feedwater. The vapor generator includes means defininga main flow path having vapor-generating tubes and means for maintainingthe vapor-generating tubes in contact with the liquid sodium. There isalso provided means for supplying feedwater to the main flow path suchthat a two-phase mixture of steam and water is passed through the mainflow path, and vapor separation means in flow communication with themain flow path for receiving the two-phase mixture and separating thesteam from the water. Regulating valve means are operatively connectedbetween the means for supplying feedwater and the main flow path andresponsive to the feedwater flow in the main flow path such that theregulating valve means will move from an open position to a closedposition in accordance with a decrease in the flow of the feedwater. Acheck valve circuit means is operatively connected in flow communicationbetween the vapor separation means and the pressure-reducing valve meanssuch that the water from the vapor separation means is passed to thevapor-generating tubes whereby natural circulation flow is establishedin the main flow path.

BRIEF DESCRIPTION OF THE DRAWINGS The above brief description, as wellas further objects, features, and advantages of the present inventionwill be more fully appreciated by reference to the following detaileddescription of a presently preferred but nonetheless illustrativeembodiment in accordance with the present invention, when taken inconnection with the accompanying drawings wherein:

FIG. 1 is a diagrammatic illustration of a combined nuclear reactor andonce-through vapor generator system; and

FIG. 2 is an enlarged sectional view showing in details the once-throughand natural circulation circuitry in accordance with the presentinvention.

2 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings andmore particularly to FIG. '1, hot liquid sodium is delivered to a vaporgenerator I0,'of the once-through type, through a sodium inlet line 12,and the liquid sodium, which has undergone heat transfer, emerges fromthe lower portion 16 of vapor'gener'ator l0 througha sodium outlet line18. The liquid sodium in line 18 is passed by feed pump 22 to a nuclearsource 24 which includes an intermediate heat exchanger 26 and nuclearreactor 28. The hot liquid sodium is circulated through line 30 intoindirect heat exchange with coils 34 of the intermediate heat exchanger26 and line 36 and pump 38 are provided forreturning the liquid sodiumto the nuclear'rea'ctor 28.

As best shown in FIG. 2, the vapor'generator I0 is provided with aseparator drum 40 for switch over to natural circulation operation. Theworking fluid, tobe henceforth referred to as feedwater, is forced bypump means at the feedwater heaters which are not illustrated through aline 42, to a junction designated 46 where -a portion of the feedwateris carried along a line 48 to a regulator valve 50. A line 52 from valve50 is connected at a junction 53 to a line 54 which in turn is connectedto vapor generator 10. The regulator valve 50 serves as a means formatching the feedwater flow with the steam output demand ofvaporgenerator l0. Vapor generator 10,- as illustrated in FIG. 2, may becomprised of a'parallel multiple tube serpentine configuration such asdescribed and claimed in copending application Ser. No. 743,679,entitled Sodium Heated Steam Generator", and filed on July 10, I968.

The vapor generator 10 includes a pressure vessel 56 in which there isprovided a main flow path comprising an inlet header 58, downcomer 60,serpentinetubes 66 and an outlet header 68. Upon entering the header 58from line 54, the feedwater flows through downcomer 60 into theserpentine tubes 66 and isbrought into indirect heat exchange with thehot liquid sodium. A two-phase mixture of steam and water flows upwardlyin serpentine tubes 66 and emerges as superheated steam in the outletheader 68.

The separator drum 40 is connected by line in flow communication withoutlet header 68 for receiving the superheated steam. The bottom end ofseparator drum 40 is connected by a line 70 to a conical nozzledeflector 72. The steam in line 70 is conveyed into nozzle deflector 72,the upper portion of which is 'provided with a group of mechanicalseparators 73 that are well known in the art. A series of dryers 74 arepositioned above the mechanical separators 73, such that the steampasses through the dryers 74 prior to passingthrough outlet 75 to asuperheater and steam user not shown in the drawings. The water frommechanical separators 73 and dryers 74 flows downwardly in the separatordrum 40 to form a water reservoir 76 below deflector nozzle 72.Additional water is supplied to the reservoir 76 through line 78 whichis provided with a feedwater valve 80. The normal water level requiredin separator 40 is indicated on a gauge 84, and this level is maintainedby mechanical or electrical systems well known in the art which areconnected to valve 80, as schematically shown by line 86. An outletoverflow valve 88 is connected to the bottom of separator 40 throughwhich excess water is directed, thereby preventing overflowing throughdeflector nozzle 72. The feedwater valve is connected to an auxiliarysupply of feedwater by means of line 89, pump 90 and valve 91.

To establish natural circulation flow in the vapor generator 10, anonreturn check valve 92 is connected through line 94 between thebottomof separator 40 and junction 53. At fullload operation, withonce-through flow in vapor generator 10, the regulator valve 50 will bein a fully open position. During low-load operation of vapor generator10, the regulator valve 50 will be in a partially open position indirect response to the low feedwater flow rate in line 48, and this willresult in a relatively low pressure in line 52. This low-pressurecondition will cause check valve 92 to open due to the hydrostatic headdeveloped above valve 92 by the water reservoir 76.

The hydrostatic head of reservoir 76 in cooperation with the mechanicalseparation shroud from the mechanical separators 73 which act as a girthbaffle, will induce natural circulation in vapor generator 10. In thismanner, the water from reservoir 76 will flow downwardly through line94, check valve 92 and line 54, which will serve as a downcomer tocomplete the natural circulation flow loop during low-load operation.When natural circulation flow has been fully established, the regulatorvalve 50 can be moved to a fully closed position and the vapor generatorwill temporarily utilize the feedwater from the reservoir 76 until asuitable additional source can be obtained. The additional feedwater canbe obtained from the once-through supply line 42 or from the auxiliarysupply of line 89.

From the foregoing it can be appreciated that the vapor generator 10achieves continuous operation at full-load to low-load operatingconditions. Accordingly, when it is necessary to close down the nuclearreactor 28 and heat transfer occurs for a lead period which cannotsupport once-through operation in vapor generator 10, it is possible toswitch over to natural circulation flow and maintain a continuousvaporgenerating sequence. The ability to change over to naturalcirculation at low-load operation is a safety feature which is desirablefor nuclear systems combined with once-through heat transfer circuitry,since continuous flow attendant with cooling will be assured even thougha failure occurred which would eliminate the once-through flow.

A latitude of modification, change and substitution is intended in theforegoing disclosure and is some instances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that the appended claims beconstrued broadly and in a manner consistent with the spirit and scopeof the invention herein.

What is claimed is:

l. A once-through vapor generator capable of operation under variableload conditions and achieving indirect heat exchange between liquidsodium and feedwater comprising means defining a main flow pathincluding vapor-generating tubes,

means for maintaining said vapor generator tubes in contact with saidliquid sodium,

means for supplying feedwater to said main flow path such that atwo-phase mixture of steam and water is passed through said main flowpath,

vapor separation means in flow communication with said main flow pathfor receiving said two-phase mixture and separating the steam from thewater,

regulating valve means operatively connected between said means forsupplying feedwater and said main flow path and responsive to thefeedwater flow in said main flow path such that said regulating valvemeans will move from an open position to a closed position in accordancewith a decrease in flow of said feedwater, and

check valve circuit means operatively connected in flow communicationbetween said vapor separation means and said regulating valve means suchthat said water from said vapor separation means is passed to saidvapor-generating tubes whereby natural circulation flow is establishedin said main flow path.

2. A once-through vapor generator capable of operation under variableload conditions and achieving indirect heat exchange between liquidsodium heated by a nuclear reactor and feedwater comprising meansdefining a main flow path including vapor-generating tubes,

means for maintaining said vapor generator tubes in contact with saidliquid sodium;

means for supplying feedwater to said main flow path such that atwo-phase mixture of steam and water is passed through said main flowpath,

vapor separation means positioned above and in flow communication withsaid main flow path for receiving said two-phase mixture and separatingthe steam from the water to form a hydrostatic head above said mam flowpath,

regulating valve means operatively connected between said means forsupplying feedwater and said main flow path responsive to the feedwaterflow in said main flow path such that said regulating valve means willmove from an open position to a closed position in response to adecrease in flow of said feedwater, and

check valve circuit means operatively connected in flow communicationbetween said vapor separation means and said regulating valve means suchthat said hydrostatic head will cause said water from said vaporseparation means to pass to said vapor-generating tubes whereby naturalcirculationflow is established in said main flow path.

3. A once-through vapor generator capable of operation under variableload conditions and achieving indirect heat exchange between liquidsodium heated by a nuclear reactor and feedwater comprising meansdefining a main flow path including vapor-generating tubes,

means for maintaining said vapor generator tubes in contact with saidliquid sodium,

means for supplying feedwater to said main flow path such that atwo'phase mixture of steam and water is passed through said main flowpath,

a separator drum positioned above and in flow communication with saidmain flow path for receiving said two-phase mixture and for mechanicallyseparating the steam from the water to form a shroud of water in saidseparator drum, and a hydrostatic head above said main flow path,

regulating valve means connected between said means for supplyingfeedwater and said main flow path responsive to the feedwater flow insaid main flow path such that said regulating valve means will move froman open position to a closed position in direct proportion to and inresponse to a decrease in flow of said feedwater, and

check valve circuit means operatively connected in flow communicationbetween said vapor separation means and said regulating valve means suchthat said hydrostatic head in cooperation with said shroud of water willinduce natural circulation flow in said main flow path.

1. A once-through vapor generator capable of operation under variableload conditions and achieving indirect heat exchange between liquidsodium and feedwater comprising means defining a main flow pathincluding vapor-generating tubes, means for maintaining said vaporgenerator tubes in contact with said liquid sodium, means for supplyingfeedwater to said main flow path such that a two-phase mixture of steamand water is passed through said main flow path, vapor separation meansin flow communication with said main flow path for receiving saidtwo-phase mixture and separating the steam from the water, regulatingvalve means operatively connected between said means for supplyingfeedwater and said main flow path and responsive to the feedwater flowin said main flow path such that said regulating valve means will movefrom an open position to a closed position in accordance with a decreasein flow of said feedwater, and check valve circuit means operativelyconnected in flow communication between saiD vapor separation means andsaid regulating valve means such that said water from said vaporseparation means is passed to said vapor-generating tubes wherebynatural circulation flow is established in said main flow path.
 2. Aonce-through vapor generator capable of operation under variable loadconditions and achieving indirect heat exchange between liquid sodiumheated by a nuclear reactor and feedwater comprising means defining amain flow path including vapor-generating tubes, means for maintainingsaid vapor generator tubes in contact with said liquid sodium; means forsupplying feedwater to said main flow path such that a two-phase mixtureof steam and water is passed through said main flow path, vaporseparation means positioned above and in flow communication with saidmain flow path for receiving said two-phase mixture and separating thesteam from the water to form a hydrostatic head above said main flowpath, regulating valve means operatively connected between said meansfor supplying feedwater and said main flow path responsive to thefeedwater flow in said main flow path such that said regulating valvemeans will move from an open position to a closed position in responseto a decrease in flow of said feedwater, and check valve circuit meansoperatively connected in flow communication between said vaporseparation means and said regulating valve means such that saidhydrostatic head will cause said water from said vapor separation meansto pass to said vapor-generating tubes whereby natural circulation flowis established in said main flow path.
 3. A once-through vapor generatorcapable of operation under variable load conditions and achievingindirect heat exchange between liquid sodium heated by a nuclear reactorand feedwater comprising means defining a main flow path includingvapor-generating tubes, means for maintaining said vapor generator tubesin contact with said liquid sodium, means for supplying feedwater tosaid main flow path such that a two-phase mixture of steam and water ispassed through said main flow path, a separator drum positioned aboveand in flow communication with said main flow path for receiving saidtwo-phase mixture and for mechanically separating the steam from thewater to form a shroud of water in said separator drum, and ahydrostatic head above said main flow path, regulating valve meansconnected between said means for supplying feedwater and said main flowpath responsive to the feedwater flow in said main flow path such thatsaid regulating valve means will move from an open position to a closedposition in direct proportion to and in response to a decrease in flowof said feedwater, and check valve circuit means operatively connectedin flow communication between said vapor separation means and saidregulating valve means such that said hydrostatic head in cooperationwith said shroud of water will induce natural circulation flow in saidmain flow path.